Project Summary Schistosomes are parasitic worms that obligately cycle between human hosts and snail vectors. Transmission dynamics vary greatly among populations of snails, causing variability in human risk of exposure. Over 200 million people are currently infected with schistosomes, making it the second most detrimental Neglected Tropical Disease following malaria. There is no vaccine to prevent infection, and drug treatment of infected patients is limited by emerging drug resistance and the immediate potential for re-infection following treatment. Therefore, one tool used to prevent infections in humans is biological control of snail populations. One approach to biological control is the introduction of non-vector snails that outcompete vectors for food resources. This strategy has been effective in some regions and failed in others. In principle, non-vector snails should decrease transmission potential of schistosomes to people by diverting searching parasites away from appropriate vectors (a ?decoy effect?) and by decreasing the abundance of vector snails. We hypothesize that the body size of vector and non-vector snails may be an important yet overlooked trait that explains variable outcomes in snail biological control. This proposal aims to address these gaps by combining experiments and mathematical models of schistosome transmission dynamics. The results of this project could help explain the current shortcomings of biological control with non-vector snails, suggest conditions or strategies that may improve disease control, and build a framework to evaluate how future snail introductions could affect human exposure to schistosomes. We will focus on Schistosoma mansoni and Schistosoma haematobium, the two species of schistosome that together cause the majority of human infections, which only infect snails in the genera Biomphalaria and Bulinus, respectively. We will use Melanoides as a non-vector competitor snail because it has previously been intentionally introduced to schistosome endemic regions with mixed outcomes. In Specific Aim 1, we will use a novel parasite fluorescent-labeling bioassay to determine the vector traits that influence exposure and susceptibility to S. mansoni and S. haematobium and build a mathematical model to predict the impact of non-vector and vector body size upon schistosome transmission in multi-species communities. In Specific Aim 2, we will test these predictions using experimental schistosome epidemics in snail communities that vary in species composition. This work will allow us to determine the ecological context in which snail control can aid in human schistosomiasis prevention. Furthermore, our data will allow us to rigorously test how non-vector snails influence schistosome dynamics in order to guide public health interventions and management strategies in at-risk regions. Understanding snail ecology is a key step in combatting this devastating disease worldwide.